1. Field of the Invention
The present invention relates to foreign matter checking in which particles of foreign matter on a semiconductor substrate are detected.
2. Description of the Related Art
In recent years, as integrated circuit technologies have come to achieve higher and higher levels of circuit integration, the patterns of elements formed on semiconductor substrates have become more and more minute. In these circumstances, when there are any particles of foreign matter on the surface of a semiconductor substrate, a minute pattern cannot be formed properly, resulting in a reduced yield of integrated circuits. Accordingly, it is necessary to check for fine particles of foreign matter on semiconductor substrates, and to control the number, the size and the distribution of such foreign particles.
Hitherto, a system known as a laser scattering system has been employed to detect particles of foreign matter on semiconductor substrates. In this system, a laser beam from a laser source is projected onto the surface of a semiconductor substrate. If there is any foreign particle on the semiconductor substrate, a portion of the laser beam is scattered. Such scattering of the laser beam is detected to check foreign matter on the substrate (see Japanese Patent Unexamined Publication No. 3-225939).
In the conventional system for checking foreign matter by utilizing laser beam scattering, the light source is one that generates light of a single wavelength. As a result, when the substrate subjected to foreign matter checking is a multi-layer thin film substrate having a film of a plurality of layers deposited thereon, light scattered by a foreign particle causes interference, as shown in FIG. 5, and such interference creates some detection sensitivity-reduced regions in the deposited film in which region the capability for measuring the size of foreign particles deteriorates. Moreover, the thickness of films deposited in a semiconductor process may subtly vary among a plurality of thin film semiconductor substrates, thereby causing variations in the sensitivity with which foreign particles on the substrates can be detected.